1. Field of the Invention
The invention is based on a metering device for introducing liquid reducing agent into an exhaust gas tract.
2. Description of the Prior Art
In the operation of internal combustion engines, exhaust gases are produced, which contain nitrogen oxides. For reducing the nitrogen oxides, liquid reducing agent is used, which reduces NOx to N3 and H2O. To that end, the liquid reducing agent is introduced into the exhaust gas tract of the engine via a metering valve.
With typical liquid reducing agent there is the danger that it will freeze at low temperatures. Depending on the antifreeze agent used, the freezing point is between −11° C. and approximately −40° C. At the phase transition of the liquid reducing agent to the solid phase, an increase of up to 10% in volume can occur, so that the components of the metering device must have a suitable resistance to ice pressure. Alternatively, other measures are employed for protecting the affected components from freezing of the reducing agent. The resistance to ice pressure of the metering valve, which injects the liquid reducing agent into the exhaust gas tract, is of particular relevance for the function of the metering device.
In utility vehicles, the additional line that serves to actively cool the metering valve with reducing agent is used for ventilation. In that case, the additional line serves to aspirate air from the air space in the tank and to use it for ventilating the metering device. As a result of the ventilation, the liquid reducing agent is removed from the individual components, and as a result, if freezing occurs, no damage ensues. However, in that system two lines are required, one supply line and the additional line, which is also used for the active cooling. In the passenger car, for example, passive cooling is employed, and thus no second line is available.
In German patent disclosure DE 10 2004 054 238 A1, a metering system is described in which a feed pump for reducing agent furnishes reducing agent to a metering valve in a feeding direction, while in the reverse feeding direction it aspirates fresh air via a valve, in order the ventilate the metering system. For delivering the fresh air, an additional controllable ventilation valve is employed.